@Max Million Excellent Questions!
1) It really depends on the character. I started off with the age range being between 10-16 years old, but the need for variety has become very apparent. If your character is someone who is just starting out, I would say put them in that age range. If they've been at it a while, the sky is pretty much the limit.
2) Yes, characters can have Pokémon prior to the regional starters. While I don't expect Ash Ketchum himself to appear in this region, you could start with a Pikachu or a another Pokémon. All I ask is that you try to keep the power scaling somewhat consistent. It's no fun for anyone if you keep creaming every trainer and opponent that comes along with your Hyper Trained team. {Side note: This region actually has an unofficial 4th starter, like Pikachu in Pokémon Yellow. I haven't added it to the Pokédex yet, but its based on a Pokémon cut from the Johto games called Berunrun.}
3) It is possible. However, I was saving that for the post-game content. There are 12 locations where you can obtain a starter Pokémon and, as of this region, 12 sets of starters. During the post-game, people who complete the Laborer's Challenge can obtain 1 Pokémon from all the current Pokémon generations as a reward for passing a trial.
4) I can make it work. I've actually been rewriting the teams a little to make them appear more like game antagonists.
5) I've been controlling the gym leaders and professors up to this point. However, if you only need to ask them a quick question, shoot them an email, or have a phone conversation, you may play as them temporarily. Unless you want to create a character who is a professor ora gym leader, then you could play as them full time. It would take some adjusting, but I could make it work.
No worries about the questions! You raise some good points, and I'm excited for your interest.